The roller hearth kiln is a tunnel-shaped furnace in which a multiplicity of rollers is disposed horizontally and parallel to one another. The materials to be fired are conveyed on the rollers from the inlet to outlet of the kiln so as to be passed through the preheating, firing and cooling zones thereof.
When firing ceramic building materials, such as tiles, bricks or roof tiles, in a roller hearth kiln, it has hitherto been usual practice to feed them one after another through the kiln. It has been usual that each material to be fired has a residence time of, say, one to two hours in the kiln (the time between its charging in the kiln at the inlet thereof and its discharging therefrom at the outlet thereof).
Such a short residence time is one of the advantages arising from the use of a roller hearth kiln, and contributes to rapid firing.
The residence time as stated above is, however, so short that a roller hearth kiln can be used only for firing a material comprising a body and glaze which can be rapidly fired. Moreover, the time is too short for producing a deep color by reduction or oxidation on the material to be fired.